1. Field of the Invention
This invention relates to an electrolytic process for the preparation of tetraalkyl 1,2,3,4-butanetetracarboxylates from dialkyl maleates. The products are useful as precursors of the corresponding free acid, 1,2,3,4-butanetetracarboxylic acid. Conversion of the tetraalkyl 1,2,3,4-butanetetracarboxylates into the corresponding free acid can be effected as described and claimed in commonly assigned U.S. Pat. No. 5,298,653. This reference is herein incorporated by reference.
The corresponding free acid, 1,2,3,4-butanetetracarboxylic acid, has been found by the U.S. Department of Agriculture to be an effective permanent press agent for polyester-cotton blend fabrics, and could find use in large quantities for such purpose. Accordingly, an efficient process for the preparation of the free acid is deemed highly desirable and useful. A requirement of any such process, however, is that it must produce a product exhibiting acceptable color performance properties, as this is a critical factor for suitability for permanent press agents.
2. Description of the Related Art
Electrolytic reductive couplings of various activated olefins have been investigated and reported in the art. Much of this work involved aqueous systems in a divided cell, and often with a supporting electrolyte salt with a very negative discharge potential, such as a quaternary ammonium salt. In addition, however, to the desired reductive coupling reaction, other undesired side reactions such as, for example, simple reduction and polymerization frequently occur. Various parameters of such reactions have been discussed, including the use of various supporting electrolytes. See Organic Electrochemistry, 2nd ed, Baizer and Lund, Ed., Marcel Dekker, Inc., New York, N.Y., 1983. At page 669 of this reference, for example, it is stated that undivided cells are operable with the restrictions that (i) the olefin and reaction product not be substantially oxidized at the anode, and (ii) the oxygen evolved at the anode in aqueous systems not promote undesirable side reactions. In addition, at pages 669 and 672, reference is made to dimerization of diethyl maleate and the effect of alkali metal cations in increasing the rate of dimerization of anion radicals.
Electrolytic hydrodimerization, also referred to as electrohydrodimerization, of diethyl maleate has been reported by Baizer et al, Journal of the Electrochemical Society, 114(10), 1024-1025 (1967). In accordance with the described procedures, the electrolyses were carried out using a catholyte of water and dimethylformamide in a divided electrolysis cell. The reference further indicated that, all other conditions being equal, more hydrodimerization occurs in the presence of tetraethylammonium ion than of sodium ion. The electrolyses were carried out for three (3) hours, generally resulting in about 50 conversions, and specified amounts of hydrodimer, and other products.
Methanol has been employed as a solvent for the study of reduction mechanisms. In Sazou et al, Collections of Czechoslovakia Chemical Communications, 52, 2132-2141 (1987), cyclic voltammograms of dilute methanol solutions--for example, 0.0025 or 0.005 mole/liter--of maleic acid and fumaric acid with various supporting electrolytes, employing a hanging mercury electrode, are presented, and reduction mechanisms discussed. The reference postulates that the double bond reduction of the corresponding dimethyl esters of maleic acid and fumaric acid occurs in one step.
Electrohydimerization of dialkyl maleates is known in the art. In U.S. Pat. No. 5,244,546, a process is described for the electrolytic reductive coupling of dialkyl maleates to yield tetraalkyl 1,2,3,4-butanetetracarboxylates. In accordance with the process, the electrohydrodimerization is carried out by subjecting an electrolysis medium comprising a substantial concentration of the dialkyl maleate in a substantially anhydrous alkanol, and a supporting electrolyte to electrolysis in an undivided electrolysis cell. The reaction reportedly results in good yields of tetraalkyl 1,2,3,4-butanetetracarboxylates.
In many instances, however, particularly in a commercial scale process, a small percent increase in the yield of the desired product, relative to known processes, represents a tremendous economic advantage. Accordingly, research efforts are continually being made to define new or improved processes for preparing new and old desired products. The discovery of the process of the instant invention, therefore, is believed to constitute a decided advance in the electrohydrodimerization art.